This invention relates to the fabrication of optic fibers and, in particular, to the fabrication of glass xe2x80x9cpreformsxe2x80x9d from which optic fibers are formed by known fiber pulling and drawing processes.
This application incorporates by reference the teachings of application Ser. No. 09/427,393 U.S. Pat. No. 6,349,606 entitled xe2x80x9cReduced Backlash Translating Systemxe2x80x9d filed this Oct. 25, 1999 simultaneously with the filing of this application.
Known systems and processes for forming glass preforms suffer from certain problems which may be better explained by reference to FIG. 4. FIG. 4 shows a preform tube 10 (also referred to as a xe2x80x9cstarter tubexe2x80x9d) used to form optic fibers having an inlet end 11 and an exhaust end 13. Reactants vapors and gases are introduced at the inlet end 11 of the starter tube 10 and flow through the tube which is being rotated. As the chemical vapors flow through the starter tube 10 an oxygen-hydrogen torch 14 slowly travels from left to right (inlet end 11 to outlet end 13) and then from right to left (outlet end 13 to inlet end 11), along the length of the tube. The chemical vapor mixture undergoes a chemical reaction in the hot zone, near the torch, in which the incoming vapors form oxides. The chemical reactions produce microscopic glassy particles or xe2x80x9csootxe2x80x9d that collect on the inside surface of the starter tube downstream from the hot zone and take the appearance of a thin porous sooty layer. Byproducts of the reaction together with the soot that does not deposit on the tube, flow out of the exhaust end of the tube. As the torch moves along the tube, the intense heat it generates causes the individual soot particles to be sintered into a continuous glassy layer. The torch is caused to pass back and forth along the length of the starter tube to cause layers upon layers to be formed, until the starter tube is filled.
In order to ensure that the layers formed are uniform, the torch must be moved along the full length of the starter tube in a very controlled, continuous, manner. However, several problems exist in achieving this goal, particularly at the ends of the tube, which may be best described as follows. As the torch is moved between the two ends of its travel it is repeatedly decelerated and brought to a stop at one end and then accelerated to a selected uniform speed until it comes close to the other end at which point it is decelerated and stopped at the other end before the process is repeated. Thus, at each end of travel, there is a problem due to the non-uniform application of heat to the tube. In addition, any momentary hesitation (xe2x80x9cbacklashxe2x80x9d) in the travel of the torch, for whatever reason, causes more heat to be generated at the point of hesitation resulting in a non-uniformity in the glass layer at that point. The non-uniformity (difference) may render defective and useless the entire starter tube. When it is realized that many hours (e.g., 6 hours) may be required to fill a starter tube and that significant material and labor costs are associated with the manufacture of each tube, it becomes clear that having to discard a starter tube, even when partially filled, results in significant economic and energy losses.
Applicants"" invention is directed to reducing defects occurring in the fabrication of starter tubes due to hesitation and backlash of the heat source as it travels along the length of the tubes.
A system for fabricating a preform in accordance with the invention includes supplying reactants and gas vapors to the preform and the use of two opposing forces to control the movement of a heat source supplying heat to the preform. The two forces function to reduce any hesitation or backlash in the movement of the heat source along predetermined portions of the preform to enable uniform layers to be formed within the preform.
In one embodiment of the invention, a system for forming a glass preform tube in accordance with known fabrication processes includes two opposing forces which are simultaneously applied to first and second motorized assemblies for causing movement of the heat source axially back and forth along the tube. The actual rate of movement of the heat source is a function of the two forces. Along the length of the tube between the ends of the path of travel of the heat source, the effect of the forces is to preferably provide a constant first amplitude for a fixed rate of travel of the heat source corresponding to a first fixed flow rate of reactant chemicals through the tube. At the ends of the path of travel of the heat source, the amplitudes of two opposing sources are varied relative to one another such that the sum of the two forces first decreases to zero in accordance with a preselected deceleration function and then increases from zero to the first amplitude in accordance with a preselected acceleration function, such that the heat source is applied uniformly over the length of the preform tube.
In a system embodying the invention, simultaneously with the variation of the two forces, the rate of flow of the reactant chemicals is first decreased and then increased according to flow rate variation functions corresponding to the deceleration and acceleration functions of the sum of forces, respectively. Preferably, each of the opposing forces never decreases to a value below a threshold value sufficient, by itself, to overcome standing friction of the heat source and for causing movement of the heat source from a stationary condition.
In a system embodying the invention, two opposing forces are applied to first and second motorized assemblies whose movements are sensed for controlling the movement of the motorized assemblies and the profile of the reactants and vapor gases supplied to the preform tube.